Flame-based detection is a common technique used in chromatography (e.g., gas chromatography) to detect analytes of interest (e.g., organic compounds) in an analyte stream. For instance, flame ionization detection (FID) functions by maintaining a flame via the addition of a combustible fuel (e.g., hydrogen) and an oxidant (e.g., oxygen) to the detector. An analyte stream (e.g., the eluent from a gas chromatography column) passes through the flame in the flame ionization detector. Compounds that contain a reduced form of carbon (e.g., organic compounds from the analyte stream) are ionized in the flame to produce carbon-based ions and free electrons. Compounds without reduced carbon such as carbon dioxide, nitrogen and noble gases (e.g., helium) do not. The newly-generated free electrons are attracted to a positive electrode (e.g., anode) while the carbon-based ions are attracted to a negative electrode (e.g., cathode). As the ions and electrons reach their respective electrodes, an electric current is established. The amount of current flow is thus proportional to the number of reduced carbon atoms entering the flame ionization detector. Accordingly, flame ionization detectors are very selective for, and can accurately measure the presence of, analytes of interest that contain carbon (e.g., organic compounds).
Flame photometric detection (FPD) is another technique used in chromatography (e.g., gas chromatography) to detect analytes of interest (e.g., small organic or organometallic compounds) in an analyte stream. FPDs also maintain a flame supplied by a combustion gas (e.g., a combustible fuel and/or an oxidant). Instead of measuring current generated via the ionization of carbon-based compounds, flame photometric detectors measure the light emitted when analytes (e.g., compounds capable of chemiluminescence in a flame) combust. In some cases, certain atoms (e.g., sulfur, phosphorous, tin, chromium or tellurium) can emit light at specific wavelengths upon combustion, thus enabling a practitioner to selectively determine the presence of a given analyte of interest by measuring for excitation at a specific wavelength unique to an atom known to be present in the analyte of interest.
In general, there are a number of different mobile phase fluids used in chromatography. Various chromatographic systems can use different mobile phase fluids depending on the nature of the separation to be carried out. For instance, liquids (e.g., acetonitrile), gases (e.g., helium), or compressible fluids, (e.g., carbon dioxide) can serve as a mobile phase. In addition, when employing a compressible fluid as a mobile phase, the density of the mobile phase can be increased or decreased over the course of a chromatographic separation while the volumetric flow rate is kept constant (e.g., a density-programmed gradient separation).
In chromatographic systems using flame-based detection, the use of density programming can interfere with the detection of analyte molecules. For instance, the mass flow rates of combustible fuels and/or oxidants (e.g., hydrogen and/or oxygen) to a flame-based detector can be optimized according to the mass flow rate of mobile phase fluid (e.g., carbon dioxide) at the beginning of a separation. A change in mass flow rate of mobile phase fluid during the separation resulting from the use of density programming can cause instability in the flame and decreased detector performance. In an extreme example, the flow rates of the combustible fuels and/or oxidants that were optimized for best response at the beginning of a separation can be inadequate to maintain a stable flame at some point later in the separation if a density program is used.